Bistable cholesteric liquid crystal displays were introduced in the early 1990's (see U.S. Pat. Nos. 5,437,811 and 5,453,863). Their zero-power image retention and sunlight readability led to their integration into numerous signs and battery-powered applications as reviewed in “Cholesteric Liquid Crystals for Flexible Displays” in Flexible Flat Panel Displays, Ed. G. Crawford, (John Wiley & Sons, 2005) J. W. Doane and A. Khan, Chapter 17. The technology is best suited for reflective color images. In the cholesteric display technology, multiple-color and full-color displays are preferably produced by stacking multiple cholesteric liquid crystal layers with each tuned to reflect a different wavelength, typically red, green, and blue (see U.S. Pat. No. 6,654,080). These three colors are additively mixed to achieve up to eight colors. Images with more colors are possible because the technology is amenable to grayscale. That is, the reflective brightness of each color can be electronically adjusted to any desired level between the display's maximum and minimum brightness. Each level of brightness is referred to as a gray level. The total number of colors depends upon the number of gray levels one can choose for each color layer. High resolution displays with as many as 4096 colors have been produced.
Commercial bistable cholesteric displays of the prior art display digital images and as such are made using of a matrix of pixels with each of the pixels having a small area. The resolution of the display depends upon the number of pixels and size of the display. Typical pixel sizes are substantially less than one square millimeter. These displays are typically manufactured on glass substrates. Recent progress has been made in commercializing displays built on flexible plastic substrates rather than glass. The new flexible displays are manufactured with a simple lamination process, and may be cut into interesting shapes after assembly. Of significance, these displays are very thin since thin plastic sheet material as thin as 12.5 microns can be used for the substrates making possible a display with the over all thickness less that 60 microns. Using cholesteric liquid crystals dispersed as emulsified droplets has made possible even thinner displays since all the materials of the display including the electrodes, substrates and cholesteric dispersion can be coated in thin layers.
Such developments suggest a display film that can be electronically switched from one color to another color that can be laminated to flat surfaces and even made to conform to curved surfaces in the form of a skin. Consumers frequently identify color as a necessity for several types of products, such as; clothing, accessories, hand held electronics such cell phones, personally worn electronics, medical indicators, and decorative items. The color on these items is defined on the product when purchased. Conventionally, it has not been possible to electronically change the color of these items after the initial purchase. Thin flexible displays for changing the color of articles, for example, an electrochromic layer or a cholesteric display skin for changing the color of cell phones, have been described in the patent literature but such devices have not been successfully implemented (Published Patent Application No. 2008/0074383 and U.S. Pat. No. 7,142,190). Such cholesteric display skins would suffer from a problem of gray scale discontinuity discussed below. Other products incorporate a color change indicator for either a sensorial signal to indicate the product is properly working or to indicate the user's attention is required. Several color indicator products exist such as battery testers (U.S. Pat. No. 7,188,996) and self expiring security badges (U.S. Pat. No. 6,752,430).
Cholesteric display films have not been suitable for electronic skin applications with tunable uniform colors because uniform gray levels have not been possible in areas around one square centimeter and larger. In areas of such size, the inventors have noticed that levels of gray become very non-uniform or blotchy in appearance. The reason for this is not completely understood but it is believed by the inventors to be a result of several possible causes such as: non-uniform cell gap thickness (varying distance between electrodes) and non-uniform conductivity of transparent electrodes. Such features have not been a problem in typical cholesteric matrix displays because the pixels are so very small that gray levels appear uniform on the scale of a pixel and image content makes the non-uniformity hard to detect across many pixels. Furthermore, nearly all commercial displays have been driven in a binary (on/off) mode not utilizing shades of gray.
Cholesteric Liquid Crystal Display Prior Art:
Prior art bistable cholesteric display technology has been used on digital displays incorporating a matrix of small pixels. The most suitable matrix for the cholesteric technology has been the passive matrix because of a unique voltage threshold in its electrooptic voltage response curve. A passive matrix is a plurality of row electrodes on one substrate and a plurality of column electrodes on the opposing substrate orthogonal to the row electrodes. Intersection of the row and column electrodes forms a matrix of pixels of the liquid crystal material between the electrodes. As the electrode rows and columns are very thin in such passive matrix displays, each of the pixels has a very small area, of the order of a square millimeter or much less, providing the display with high resolution. Row and column voltage waveforms are applied to the pixels using row and column driver circuitry, changing the reflective states of the pixels. The voltage threshold characteristic of a bistable cholesteric material allows the pixels in each row to be independently addressed while unaffecting the others. A passive matrix display is addressed a row at a time until the entire display is addressed with an image. Because of bistability, the image is retained on the display indefinitely or until a new image is addressed on the matrix.
Referring to FIG. 1, a typical drive scheme for cholesteric displays of the prior art involves erasing the display to either a planar (bright) or focal conic (dark) texture and then driving the display to its desired brightness with a drive pulse. This figure shows the resultant reflectivity of driving a typical bistable cholesteric display cell of the prior art starting from the planar state of maximum reflectivity, curve 10, as well as from the focal conic state of minimum reflectivity, curve 11. The focal conic state is a forward scattering state that is essentially transparent to the light traveling through it. Each point in curve 10 was achieved by first applying an erase pulse (sometimes referred to as a clearing pulse; see U.S. Pat. No. 5,644,330 as well as later discussion in this document) to drive the display to its highest reflectivity in the planar texture as described in the prior art. Following each erase pulse a 100 ms drive pulse (frequency=250 Hz) is applied to achieve the recorded reflectivity. Each point in curve 11 was likewise achieved by first applying an erase pulse (sometimes referred to as a clearing pulse; see U.S. Pat. No. 5,644,330), however driving the display cell to its darkest (least reflective) focal conic state. Following each erase pulse a 100 ms drive pulse (frequency=250 Hz) is applied to achieve the measured reflectivity recorded as the normalized reflectance. The horizontal axis shows the root mean square (rms) voltage of the applied pulse.
In operation of a prior art cholesteric liquid crystal display having the electrooptic response curve shown in FIG. 1, a pulse with voltage slightly less than or equal to V3, indicated by 13, may be used to address the darkest pixels (i.e., to place pixels desired to be dark into the focal conic state). A voltage>V4, indicated by 12 on curve 10, may be used for addressing bright pixels (i.e., to place pixels desired to be bright into the planar state). Gray scale levels are those levels having a reflectivity between the planar and focal conic states. Gray scale may be achieved by using pulses with values between V3 and V4. Reflectivity of prior art cholesteric displays is highly sensitive to voltage as seen by the extreme steepness of the curve in the gray scale region between V3 and V4 (FIG. 1). Thus, the prior art gray scale drive scheme is sensitive to applied voltage and display imperfections.
Prior art drive pulses may have many forms (see for example U.S. Pat. Nos. 6,268,839 and 5,933,203). Typical but not exhaustive drive pulse forms are dc, ac, and ac with a pause between phases, illustrated in FIGS. 2 and 3. A unipolar dc pulse 21 is illustrated in FIG. 2a; an ac pulse 22 in FIG. 2b and an ac pulse 23 with a pause between in FIG. 2c. It is appreciated also that the ac examples may consist of either single or multiple periods; that is, more than one pulse, in order to satisfy drive frequency requirements of the liquid crystal materials. As shown in FIGS. 3a, 3b and 3c, in some circumstances pulse width modulation between two voltage levels (labeled V0 and V1) may be used to adjust the rms value of the drive pulses as illustration by pulses 31, 32, and 33.
Any of these prior art drive pulses can be used as drive and reduction pulses in the present invention. The width of a unipolar pulse of FIG. 2a is the distance between the leading and trailing ends of the pulse; the width of bipolar pulse of FIG. 2b is the distance between the leading end of the positive polarity pulse portion and the trailing end of the negative polarity pulse portion; the width of the bipolar pulse of FIG. 2c is the same as in FIG. 2b but includes the intervening pause at zero voltage between positive and negative polarity pulse portions; the widths of the pulse width modulated pulses of FIGS. 3a, 3b and 3c are the same as in FIGS. 2a, 2b and 2c, respectively.
U.S. Pat. No. 6,133,895 discloses a cumulative drive scheme for a cholesteric liquid crystal display for changing images on the display at a near video rate. This driving procedure takes advantage of the cumulative nature of a cholesteric display as well as its threshold characteristic to address the pixels of a passive matrix with a sequence of pulses of narrow width in the same manner as an regular liquid crystal display (LCD) in the twisted nematic (TN) or super twisted nematic (STN) mode to provide the same video image aesthetic. Because of the higher viscosity of cholesteric liquid crystals video rates are only possible however on small matrices where the number of pixels is limited. FIGS. 4A and 4B of U.S. Pat. No. 6,133,895 show increases in reflectivity and FIGS. 5A and 5B of the patent show decreases in reflectivity, using AC pulses at a constant voltage and 1 ms pulse width.